1. Field of the Invention
The present invention relates to pneumatic tires having a radial carcass structure and particularly to such tires having ground contacting tread regions which are substantially curved in axial cross-section. The invention also relates to improvements in the manufacture of such radial tires.
2. Description of Related Art
Radial motorcycle tires are one such type of tire having a highly curved tread. These tires have relatively short sidewalls extending radially and axially outward from beads with the maximum tire width being located between the edges of the tread region. The highly curved tread allows the contact area between the tire and the road surface to be maintained as the motorcycle is banked over for cornering.
In addition to tires for motorcycles there has recently been proposed in co-pending European Patent Application 94309359.1, published as EP-0658450, a radial tire for a passenger car. Unlike conventional flat-treaded car tires this new tire has a tread region having substantial curvature in transverse cross-section. In straight-ahead running the contact area between the tire tread and the road is relatively narrow in comparison to that of the conventional tire. In tests this new tire has shown improved characteristics which are believed to be due in part to the ability of the contact area to move across the tread and/or increase in axial width in response to changes in load and in cornering.
In common with conventional flat-tread radial tires the above-described highly curved treaded tires have their ground contacting tread regions reinforced by a breaker or belt which extends circumferentially around the tire radially outward and adjacent to the tire carcass. Conventionally such breakers comprise plural plies of tire fabric reinforced by parallel cords disposed at an angle to the circumferential direction, the cords of one ply being crossed with respect to the cords of an adjacent ply.
A common problem with such belted tires and particularly with curved treaded tires is so called breaker edge looseness wherein the bond between the cord reinforcement and its surrounding rubber breaks down at the edge of the breaker leading to premature failure of the tire.
It is therefore a first object of the present invention to improve the properties of radial tires of the abovementioned highly curved tread type including improving the resistance to breaker edge looseness.
Radial tire manufacture has conventionally employed a two-stage process. In the first-stage a cylindrical tire carcass is built on the cylindrical surface of a drum, the body comprising an assembly of inner liner and one or more carcass plies anchored to and interconnecting axially spaced apart annular bead cores or hoops. On completion of the first stage assembly the cylindrical carcass is removed from the drum and transferred to a second-stage machine. In the second stage of manufacture the shape of the carcass is changed from cylindrical to toroidal and then the individual breaker plies are applied sequentially to the crown of the toroid followed finally by the rubber tread strips. Such a two-stage manufacturing operation is expensive in respect of equipment, factory space, labor and time.
Single-stage manufacture of radial tires has been proposed previously. For example GB 1 569 640 discloses the single-stage assembly of a conventional flat-treaded radial tire having an additional zero degree cap band overlaying the breaker or belt. However the single-stage or so-called xe2x80x98flat buildingxe2x80x99 of a radial tire assembly necessitates a considerable increase in the circumferential length of the breaker plies when the shape of the final assembly is changed from cylindrical to toroidal. This increase in circumferential length of the breaker plies is accomplished by a complex combination of stretching of the plies and trellising of the cords. In the prior art it has been found that due to the adhesion of the breaker cords to other components of the assembly, the complex movement of the breaker plies has resulted in uncontrollable and uneven movement of the cords, distortion of the carcass ply and consequent malformation of the tire. For this reason the single-stage manufacture of a radial tire has not been adopted.
It is therefore another object of the invention to provide an improved single-stage method by which such an improved radial may be efficiently built.
According to one aspect of the invention a tire comprises a tread reinforced by a breaker assembly comprising at least two breaker plies and having in its normally inflated fitted condition a camber value C/L of between 0.3 and 0.7, a reinforcing carcass ply of cords disposed at an angle of 0xc2x0 to 20xc2x0 to the tire radial plane extending radially inside the breaker and between two bead regions to form a carcass main portion and wrapped in each bead region axially outwardly around an annular bead core to form carcass ply turn-up portions extending radially outwardly and terminating radially inward of the point of maximum tire width, characterized in that one breaker ply extends between the two bead regions having its edges disposed between the carcass main portion and the respective turn-up portion in the bead region.
By camber value is meant the ratio C/L between the radial distance C from the center to the edge of the tire tread and the axial distance L between the center and edge of the tread.
By normally inflated and fitted state is meant the state which the tire is mounted on its recommended wherein and inflated to its scheduled pressure.
According to another aspect of the invention a method of building a radial tire comprises forming a cylindrical-shaped carcass comprising axially extending carcass reinforcing cords, fitting annular bead hoops onto the radially outer surface of the carcass ply and axially inward of each of the ply edges, assembling a plurality of breaker plies centrally onto the radially outer surface of the carcass ply and co-cylindrically with it including one wide breaker ply which extends axially to a position inward and adjacent to each of the bead hoops, turning each of the carcass ply edge portions lying axially outwardly of the bead hoops radially outwardly and axially inwardly around the bead hoops to overlie the axial edges of the wide breaker ply, assembling onto the cylindrical assembly of carcass, beads and breaker, the remaining components of the tire such as a centrally disposed rubber tread flanked at either side by rubber sidewalls to form a cylindrical green tire assembly, shaping the resultant cylindrical green tire assembly into a toroid and finally molding in a heated tire mould to form the tread pattern in the tread and vulcanize the whole assembly.